In recent years, a growing demand for precious metals in high-technology applications and the increasing cost of precious metals has made the practice of recovering and refining these metals very important. To meet these demands, industry is turning to new sources of precious metals such as complex sulfide ores, and recycling precious metals from catalysts and electronic scrap.
Using traditional smelting techniques to treat these materials is not always effective due to their refractory nature and low precious metal content. Leaching with acidic chloride solutions containing an oxidizing agent is effective in removing the precious metals and has the added advantage of preserving valuable substrates for recycling. The recovery and subsequent separation of precious metals from the chloride feed liquors constitutes a difficult refining problem.
The recovery and subsequent separation of precious metals from these chloride feed liquors constitutes a difficult refining problem. These liquors generally contain low levels of precious metals (ppm levels) and high levels (on the order of grams per liter) of base metals such as iron, copper, zinc, tin, and nickel. Moreover, the volumes of solutions generated are large compared to the volume of highly concentrated solutions generated from typical precious metal refining. Classical precipitation techniques are inefficient when applied to such solutions and they are being replaced by modern separation methods. Among the latter, solvent extraction (SX) processes are being considered for the separation and extraction of precious metals by industrial companies.
Solvent extraction is sometimes referred to as liquid ion exchange extraction and it is being utilized as a promising new method in extraction and separation science. Briefly, this process is basically described by two steps. In the first, the extraction step, dilute aqueous feed solution which contains the metal ion to be recovered is mixed with an immiscible hydrocarbon diluent or carrier containing an ion exchanger or ligand dissolved therein, and the resulting metal complex migrates to the organic phase. In the second, the stripping step, the separated "loaded" organic phase is mixed with an aqueous solution of a stripping agent (e.g., sulfuric acid) and the procedure is reversed, the metal ion passing back to the new aqueous phase. As a consequence, the dilute feed solution is converted into a highly concentrated solution, from which the metal values are more readily recovered, e.g., by electrolysis. The barren organic phase is then essentially recycled through the system.
The current SX technology as it is applied to precious metals refining calls for Au, Pt, and Pd to be extracted by separate solvents or other techniques. Gold, for example, is removed first by SO.sub.2 reduction to the metal or extraction using ketones (MIBK) or esters (DBC). Palladium is then extracted using alkyl sulfides or oximes and platinum is removed using tributyl phosphate (TBP).
Despite the relatively good performance of SX processes in precious metal refining, there are some serious complications which need to be solved if SX is to be successful in separating gold, platinum and palladium from base metal rich liquors. The major drawbacks
Despite the relatively good performance of SX processes in precious metal refining, there are some serious complications which need to be solved if SX is to be successful in separating gold, platinum and palladium from base metal rich liquors. The major drawbacks to reagents used in precious metal refining operations are kinetics and selectivity. The extraction of palladium by sulfides or oximes suffers from slow kinetics. This is not an economic burden when the treated volumes are small. However, when large volumes of dilute solutions are treated the large inventory of reagent required for prolonged contact times becomes uneconomic. TBP extraction for platinum suffers from poor selectivity to other base metal chlorides and also requires high acidities to work well. The cost in acidifying large volumes of solution make this approach uneconomic.
U.S. Pat. No. 4,654,145 teaches a new technique for separation of gold, platinum and palladium based on using alkylated derivatives of 8-hydroxyquinoline. In this process the precious metals are extracted from acid chloride solutions using an alkylated 8-hydroxyquinoline such as Kelex 100. The metals are then separated using differential stripping. Platinum is stripped using water, gold is recovered by hydrolytic stripping or hydrogen reduction, and palladium is recovered using hydrogen reduction or acidic stripping. This prior art does not address the effect of base metal extraction on the differential stripping techniques employed. Iron and copper will extract with the precious metals and contaminate the water wash and hydrogen reduction steps.
U.S. Pat. No. 5,045,290 teaches the extraction and purification of platinum and palladium from base metal chlorides using alkylated 8-hydroxyquinoline. This process utilizes pH controlled scrubbing to remove acid and base metal chlorides. Platinum is recovered by a pH controlled wash and palladium is recovered by 8N HCl stripping. This prior art doe not consider gold extraction and stripping or how gold behaves during the base metal wash stages.
The prior art is not well suited to extraction of gold, platinum, and palladium present in small amounts from base metal chlorides present in large amounts. Current precious metal refining utilizes a separate reagent for each precious metal while processes using only one reagent rely on selective stripping of the precious metals. Multiple reagents and selective stripping add unnecessary complication and expense to the removal of trace amounts of precious metals.
In view of the deficiencies of the current available reagents used to extract precious metals, a more efficient and economical means must be developed. One of the objects of the present invention is to employ a single reagent to extract and a single strip stage to concentrate the precious metals after the base metals have been removed by scrubbing. This will greatly enhance the efficiency and ease of processing low grade precious metal-containing chloride solutions.